feat(igla): Wave Loop 895 — module-scope [609][2]^6 Pt non-power-of-two outer-dimension array-of-struct variable from call with indexed signed writes

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Next mechanical rung of the packed-vector array-of-struct ladder.

• Variant A: module-scope [609][2]^6 Pt variable initialized from a pure make_grid(0) call, with indexed signed field writes and assert_eq read-back inside a bench block.
• Expected: 38,976 elements, 1,247,232-bit packed vector, ~1.190 MiBit.
• Continues the post-1-MiBit boundary stress test.
• Zero compiler / reference-model / FROZEN_HASH changes expected.

Refs the Wave Loop autopilot plan.